Air bearings are known, both of the orifice type and constructed of inherently porous materials such as porous, sintered metals or ceramics, porous synthetic resins, and porous carbon. Air bearings are employed in a wide range of applications, from gas turbine engines to precision machinery, to ink jet printers, to high-speed dental drills. One field in which air bearings have received widespread use is in the robotically controlled painting of vehicles by high-speed rotary bell cup atomizers. While the invention herein may be used in a wide variety of air bearing applications, for reasons of descriptive efficiency and expedience, the detailed principles underlying the operation of the invention and the apparatus employed will be presented with a focus on its use in rotary bell cup atomizers for applying paint to workpieces.
Known rotary atomizers can include a rotating bell cup having a generally conical front flow surface extending between an inner, axially central paint supply orifice opening and a radially outer atomizing edge. Paint entering the bell cup through the central opening flows to the rear surface of a deflector and is disbursed radially outwardly towards the front diverging flow surface of the bell cup, flowing thereover to the outer edge of the cup where it is atomized to a fine mist and applied to the workpiece.
Atomization is effected by centrifugal forces produced when the cup, mounted on a central motor shaft, is rotated at high speeds, typically at 60-80,000 RPM.
Rotary bell cup atomizers are used in liquid based paint coating operations and also in powder coating operations. The invention herein described and claimed is useful in both types. In such operations, electrostatic charging is applied to the atomized particles to enhance their attraction to the grounded substrate and produce a flawless finish, all of which is well known.
An early example of the use of an air bearing to support the central rotating motor shaft of an electrostatic spray painting device is found in U.S. Pat. No. 4,368,853 (1983, assigned to Toyota, K.K.) Therein, a rotary electrostatic spray painting device is disclosed comprising a rotary shaft and a spray head fixed onto the front end of the rotary shaft. Paint is fed onto the cup shaped inner wall of the spray head. The rotary shaft is supported by a single thrust air bearing and a pair of radial air bearings. The rotating motor shaft is supported by air injected through porous air bearings which enable the shaft to rotate at high rotational speeds, said to be up to 80,000 RPM, in a substantially frictionless environment. While porous air bearings are said to be useful, the reference cites no specific material of construction of any bearing.
Illustrating the diversity of applications in which air bearings find use, U.S. Pat. No. 3,969,822 discloses a porous, static pressure air bearing device for use in a dentist's handpiece. Therein, the porous, static pressure air bearing device includes a rotatable shaft for mounting thereon a cutting tool, a bearing case, an air bearing portion provided in the bearing case for supporting the rotatable shaft, and a turbine blade formed integrally with the shaft or mounted on the shaft. The air bearing portion is formed of porous material, and the reference cites sintered porous metal or ceramics, or porous synthetic resins, as suitable bearing materials. As disclosed in the '822 reference, sleeves of inverted L-shaped or L-shaped cross section are secured to the rotatable shaft such that the sleeves are in contact with the turbine blade and mounted on the outer diameter surface of the rotatable shaft. These sleeves may be formed integrally with the rotatable shaft or manufactured separately from the shaft for convenience of working. Air supplied through the air supply passage is injected against the turbine blade to rotate the rotatable shaft with the sleeves. The shaft is said to assume a high speed of rotation because it is supported by air bearings of low friction. Since the bearing portion is supported by O-rings, the damping effect provided thereby acts to absorb vibrations occurring in the rotatable shaft during its shift from an initial rotation to high-speed rotation.
It will be seen that air bearings constructed of various combinations of porous and non-porous materials (having orifices), sleeves, and “O”-rings are disclosed in the known prior art. However, the air bearing assembly disclosed and claimed herein, which provides its inherent and significant advantages over prior bearings, all to be described hereinbelow, is nowhere disclosed in the known prior art.